A key switch is capable of generating a voltage signal when a force is applied to the switch. The resultant voltage signal may be employed in applications involving computers, alphanumeric keyboards, kitchen appliances and numerous other electronic devices.
Conventional metal contact key switches become unreliable after extended periods of use. For example, contact resistance tends to increase with age, and poor contact with the contact points produces undesirable chatter and/or bounce. In an effort to overcome the general unreliability of metal contact key switches, Hall effect key switches employing magnetic resistance elements and capacitive key switches have been developed, but each requires elaborate or complicated parts and/or controlling circuits.
More recently, poled polymeric piezoelectric films to function as the force sensing element have been developed. These films substantially overcome the deficiencies of metal contact key switches, as well as the deficiencies inherent in piezoelectric ceramics which are hard, brittle, easily broken, difficult to machine into complex shapes and often generate spurious voltage signals from sound waves striking their surfaces.
A preferred poled polymeric piezoelectric film is polyvinylidene fluoride (PVDF). By carefully controlling process steps to polarize the PVDF film, including mechanical orientation and treatment in an intense electric field, a highly piezoelectric and pyroelectric film results. Such a film is commercially available under the trademark KYNAR.RTM., a product of Pennwalt Corporation, Philadelphia, Pennsylvania, assignee of the present invention.
Kynar.RTM. PVDF film is flexible, tough and inexpensive, possesses a low modulus and a low mechanical Q factor, and hence little or no chattering. Many present day keyboards employ PVDF films as the piezoelectric or pyroelectric sensing element.
Kynar.RTM. film produces an electric charge that is proportional to applied stress (or strain) or to thermal energy. This charge represents an output signal of the film. The intensity of the output signal varies with ambient temperature of the film, the area of electrode metallization on the film and its thickness, the piezoelectric/pyroelectric constants for the film, the rate at which the stress (or strain) or thermal energy is applied, mechanical conditions (film tautness, laminations, mechanical noise and mechanical interelement coupling) and circuit conditions (shielding, impedance and leakage).
Prior art keyboard designs using PVDF film suffer a number of drawbacks, such as an excessive number of individual wire leads from the switch locations, and susceptibility to mechanical coupling and electrical crosstalk. These drawbacks do not result from using PVDF film but from unimaginative design. In addition, prior art keyboards require complicated decoding circuitry to identify precisely which one of an array of switches is actuated. This adds to the cost and complexity of such keyboards.
It is an object of this invention to provide a keyboard that reduces lead count to as few as three leads on a complex, multi-switch keyboard, thereby minimizing circuitry while providing a "smart" output signal which specifically identifies the actuated key by developing an electrical signal unique to that key